Reserve components of the United States Armed Forces

§ 10102, "the purpose of each reserve component is to provide trained units and qualified persons available for active duty in the armed forces, in time of war or national emergency, and at such other times as the national security may require, to fill the needs of the armed forces whenever, during and after the period needed to procure and train additional units and qualified persons to achieve the planned mobilization, more units and persons are needed than are in the regular components."

However, during World War II, members of the Civil Air Patrol and the Coast Guard Auxiliary were sometimes armed.

Members of the Coast Guard Auxiliary could volunteer as "temporary reservists", for duty as armed port security and harbor patrol officers.

U-boats preying on allied shipping in the Atlantic made service as a merchant mariner extremely hazardous.

State defense forces generally operate with emergency management and homeland security missions.

These forces are trained and equipped to perform specialized roles such as search and rescue, maritime patrols, augmenting state police or National Guard military police in a law enforcement role, or emergency management response.

They are typically, but not always, regionally based and recruited (unlike their active duty counterparts) and, in the case of the Army and Air National Guard, are the organized state militias referred to in the U.S. Constitution.

Members of the reserve components are generally required to perform, at a minimum, 39 days of military service per year.

The Army Reserve and Air Force Reserve are subordinated to the federal government while the National Guards are subordinated to the various state governments, except when called into federal service by the President of the United States or as provided for by law.

The latter is permitted because the National Guard are not subject to the restrictions of the Posse Comitatus Act unless they are under federal jurisdiction.